=Selective Breeding= ''(also called artificial selection)'' Selective breeding is the process of choosing which male and female individuals mate in order to develop a particular phenotypic trait. [image:] Humans have controlled the breeding of domesticated animals and plants for centuries. This practice involves breeding individuals with the most desirable phenotypes. While this works in the same way as [ natural selection], selection imposed by humans is often more rapid and intense than that occurring in nature. For instance selective breeding of corn has resulted in significant evolutionary changes (see left). The cob on the left is from an ancient corn plant and was initially purple. The modern cob on the far right is the product of thousands of years of artificial selection. Simply by crossing and growing those plants with the largest cobs, the cob size has increased over the generations. Selective breeding is a way of manipulating the transfer of genetic information from one generation to the next. It is a way of ensuring that those individuals with undesirable characteristics do not pass on the alleles for those traits, while increasing the frequency with which individuals with more favourable characteristics pass on their alleles. Over many generations breeders may be able to bring together several alleles that improve a certain characteristic. Below is an outline of the selective breeding process. Domesticated animals are known as breeds. When two different breeds are crossed they produce a crossbreed, Domesticated plants are generally known as varieties or cultivars. Cross-bred plants are generally called hybrids. ===1. IDENTIFICATION=== A breeder will first need identify what pheontypic trait(s) are desirable. This may seem simply but in many cases the desired phenotypes are not visible to the eye. For instance dairy farmers may want to increase the fat or protein content of the milk produced by their cows. ''Techniques involved:'' Identifying desirable traits may involve specialised techniques used to analyse the chemical composition of animal or plant products and their yield. ===2. SELECTION=== A breeder must then select which individuals to breed. They may base this decision on simple observable traits such as size, shape, growth rate or taste. However, techniques have been developed that now allow breeders to select individuals based on genetic differences. ''Techniques involved include:'' [ Marker Assisted Selection]. This technique is used to screen individuals for the presence of genetic markers commonly associated with desirable / undesirable traits. This has some significant advantages over traditional methods of selection. Individuals may have different markers associated with the same desirable trait. By breeding these individuals it may be possible to produce animals with a greater number of these markers and further improve the desired phenotype. Some individuals with a desirable phenotype might also possess a hidden recessive allele for a serious genetic disorder. If there is a genetic marker that is known to be associated with the recessive disease causing allele, breeders can avoid mating and carriers. Marker assisted selection involves generating a [ DNA Profile] often incorporating techniques such as: [ Restriction Enzymes], [ Ligation], [ Gene Cloning], [ PCR], [ Gel Electrophoresis] and [ Probes], ===3. BREEDING=== The selected individuals are then mated (crossed). The following ''techniques'' have been developed to improve reproductive outcomes. [ Artificial Insemination] involves transferring the semen from a male into the uterus of a female animal. Using the semen from a single male, a much lager number of females can be impregnated when compared with traditional breeding. If performed correctly, there is also a much higher chance of fertilisation. A successful artificial insemination programme relies on various other techniques to establish when females are in heat (near ovulation). '''''In Vitro Fertilisation (IVF)''''' is a technique in which sperm and egg cells are harvested and combined within a glass dish. Thus fertilisation occurs outside of the animals body. '''''Embryo Screening & Selection''''' The resulting embryos can be analysed for any defects, including genetic markers associated with certain diseases. '''''Embryo Transfer''''' is a technique in which embryos such as those produced by IVF are transferred into the uterus (womb) of an unrelated female called a surrogate. The surrogate is genetically unrelated to the embryo developing inside her. This technique is used to enable females with reproductive issues to produce offspring. Most female animals only release a very small number of eggs each cycle (e.g. human females generally only release one). Drugs can be used to stimulate the release of a much larger number of eggs (multiple ovulation). This is combined with embryo transfer (a technique called multiple ovulation embryo transfer - MOET) making it possible for a single female to produce a large number of offspring. '''''Pollen Transfer & Control''''' Plants can crossed by transferring the pollen from the flowers of one plant to another. Breeders may remove parts of the flowers to prevent any unwanted pollen transfer. ==Implications of Selective Breeding== Transgenesis can have implications on: • '''Genetic Biodiversity''' • '''Ecosystems''' • '''The health or survival of individuals''' • '''The survival of populations''' • '''The evolution of populations''' E.g. selective breeding usually reduces genetic diversity as only a select few individuals are used to produce very large numbers of offspring. This could effect the survival of the population. The precise implications are specific to each each and depend on the gene being added (or removed), the organism being modified and it's environment.
Credit: Ben Himme